1. Field of the Invention
Conventionally, a device is known which includes a logic circuit and small-capacity RAM (Random Access Memory) formed in a common substrate. In order to meet a growing demand for a memory capacity, a greater-capacity RAM is formed in the common substrate. Furthermore, as the logic circuit, use has been made of a gate array which can freely be formed by the user.
2. Description of the Related Art
In a conventional device shown in FIG. 7, large-capacity RAM4, made up of memory cell 2, timing signal generator circuit and so on, and gate array 5 (logic circuit) are formed on substrate 1. Gate array 5 is connected by metal wiring 6 to large-capacity RAM4 to provide a RAM-to-gate array interface.
If the gate array and large-capacity RAM are formed in a common substrate, there is a risk that an operation error will occur due to the interference of noise. That is, the gate array cannot, of necessity, be prepared against minority carriers and a variation in a substrate potential because the user cannot know in detail how a circuit is configured for the gate array. Furthermore, in the gate array, there is a possibility that unused gates will be left unconnected in which case the gate is electrically "floated". The unused gate is turned ON or OFF due to the occurrence of "couplings" and it may be adequately predicted that noise comes from the unused gates. The occurrence of such a noise whose level may be somewhat disregarded in an often statically-operated gate array circuit poses a greater problem in an often dynamically-operated memory circuit. If a memory is used as a dynamic memory in particular, minority carriers encounter the dropping of charges and a noise at the time of refreshing will cause a write-in error. Furthermore, a variation in substrate potential will cause a variation in threshold level and hence mistiming will occur at a timing signal generator circuit, causing a readout error or write-in error. If, as set out above, a memory and gate array are built in a common substrate, a serious situation will arises from a variation in minority carriers and in substrate potential.